The present invention relates to the sampling of process fluids such as implemented by petrochemical plants, refineries, gas separation plants, natural gas pipelines, etc, and in particular to the collection and initial conditioning of sample gas for xe2x80x9con linexe2x80x9d analyzers or filling of gas sample cylinders.
The preferred embodiment of the present invention contemplates a system configured to obtain a representative gas phase sample from a process gas containing entrained liquid, or a process gas which generally is highly susceptible to partial condensation of some gas phase components.
The preferred embodiment of the present invention teaches an assembly including a phase separation membrane and housing configured to facilitate the removal of entrained liquid from a sample gas stream. Accuracy of the sample is enhanced, and compositional changes are avoided by first extracting a sample from the process gas followed by removal of entrained liquid it may contain, with the entrained liquid removal being conducted at the prevailing process gas pressure and temperature. Thereafter the liquid free sample may be desaturated by lowering its pressure to reduce its susceptibility to condensation.
The heating value of natural gas has a significant impact on its monetary value. In general, the heating value of natural gas increases as the concentration of low volatility, high molecular weight components increases. Condensation of gas phase components, which reduce the proportion of high molecular weight components, therefore tends to decrease gas phase heating value while vaporization of entrained liquid has the opposite effect.
In order for natural gas supply to balance with demand over the next 10 to 20 years it will be necessary to increase production from deep-water fields in the Gulf of Mexico. (Refer to Volume 1, Fall/Winter 1997 official newsletter of Colorado Engineering Experiment Station Inc.) Gas produced from deep-water fields, containing higher concentrations of low volatility components such as water vapor and heavy hydrocarbons, has a higher susceptibility to condensation than shelf and onshore production gas.
Additionally, some onshore produced gas, particularly in low ambient temperature regions, frequently contain entrained liquids. Other liquids which can influence vapor phase composition when fluid pressure or temperature changes occur are glycols and amines which are carried over into the gas phase from gas contactors designed to remove water vapor and acid gases respectively.
A Joint Industry Project (JIP) is underway to address problems associated with measurement and transportation of xe2x80x9cwet gasesxe2x80x9d. A part of the JIP focus will include improvement of wet gas sampling techniques.
The American Petroleum Institute (API) and the Gas Processors Association (GPA) are two leading industry organizations having recommended standard practices for sampling and analysis of natural gas. Both recommend that entrained liquids are to be removed from natural gas samples at prevailing source gas pressure and temperature. (Refer to Manual of Petroleum Measurement Standards chapter 14-Natural Gas fluids measurement, section 1 collecting and handling natural gas samples for custody transfer, fourth edition, August 1993.) This is done to prevent gas phase compositional changes caused by vaporization and condensation.
Following the recommended practices has been almost impossible due to lack of available hardware to accomplish the task. For example GPA recommends a separator design (FIG. 6 in the aforementioned API document) which at best is suited for removal of liquid slugs and large droplets, neither, of which cause frequent sampling problems. Furthermore, there is no provision for maintaining process source gas temperature. Liquid aerosol, which are the most frequent source of liquid entrainment, are not easily separated from sample gas by this xe2x80x9cKnock-Ourxe2x80x9d type of GPA separator.
Conventional mechanical coalescer elements constructed of fibers, screens, etc. require gas flow thru the element for aerosol coalescing to occur. In most cases this precludes the return of the coalesced liquid to the process gas source at the original source pressure. With increasing environmental concerns disposal of the coalesced liquid can present serious problems if it cannot be returned to the original source.
Gas phase separation membranes are known and utilized in stack and flue gas analyzers for removal of entrained water, sub-micron aerosols, and filtration of ultra-fine particulates; examples of such membranes include the gas phase separation membranes utilized in the GENIE Series 100 line from A+ Corporation of Prairieville, La. USA. However, the utilization of said membranes is not believed contemplated in conjunction with the system of the present invention.
Unlike the prior art, the present, searched for invention provides a system for retrieving a gas phase sample from a gas stream containing entrained liquid, in a cost effective highly accurate, and generally easily implemented fashion, providing a gas sample at the prevailing process gas source pressure and temperature condition, thereby preventing gas phase composition changes. The subject invention, a sample conditioner assembly, provides a coalescer design that overcomes the problems associated with prior art systems, methods, and hardware.
The preferred embodiment of the present invention contemplates sample conditioning wherein there is provided a coalescing assembly particularly suited for coalescing, which employs a phase separating membrane for the removal of liquid entrained in sample gas.
In the first operating mode of the preferred embodiment of the present invention, this coalescing assembly is positioned external to the process source gas, and coalesced liquid separated by a phase separating membrane is returned to said process source gas by gravity free flow. In a second operating mode of this preferred embodiment of the present invention coalesced liquid, drained from the coalescing assembly, is disposed by piping to a location external to the original process source gas.
In accordance with a second embodiment of the present invention, the coalescing assembly is inserted into a housing, said housing and phase separating membrane portion of said coalescing assembly being positioned within the containment walls of a process source gas vessel or pipeline. A means is provided for inserting and withdrawing the coalescing assembly from said housing during which time said housing is inserted into a pressurized process source gas. This feature facilitates the inspection or replacement of the coalescing element which consists of a phase separating membrane.
Additionally, a valving means positioned in the lower end of said housing is actuated to a closed position by the withdrawal of said coalescing assembly which provides isolation and containment of the process source gas. Said second embodiment of the present invention has threaded means to facilitate said insertion and withdrawal of the coalescing assembly from said housing even when the process source gas is at elevated pressure.
In the third embodiment of the present invention, a pressure reducing regulator is integrated into said coalescing assembly of said second embodiment of the present invention.
In the preferred, second, and third embodiment of the present invention, the phase separation membrane employed repels aqueous and organic liquids while remaining permeable to gas.
Further, in the preferred, second, and third embodiment of the present invention, a support member, placed immediately upstream of said phase separation membrane protects the unit against physical damage which may otherwise occur if fluid flow is inadvertently reversed during a startup or shutdown operation.
A major advantage of the preferred, second, and third embodiment of the present invention over prior art, is that liquid entrained in a gas is removed at essentially the prevailing process gas source pressure and temperature condition, thereby preventing gas phase composition changes. A second advantage is that the insertion means, consisting of the rotation of a threaded member is safe, simple, and economical.
The advantage offered by the third embodiment is that pressure reduction or regulation occurs after entrained liquid has been removed thereby preventing gas phase compositional changes.
The system of the present invention may also be utilized in conjunction with conventional sampling and analyzation techniques including isokinetic sampling and gas chromatography analysis, for example, to discern the amount of entrained liquid in a gas stream. In this regard, a sample of the gas stream would be processed with the present invention to remove entrained liquid therefrom, and this data from this sample would be compared with data obtained on said gas stream utilizing traditional sampling techniques, thereby allowing the user to discern the amount and type of entrained liquids removed from the stream, thereby providing an enhanced analysis of the gas stream.
In the article Effect of Entrained Liquid on Orifice Measurement by V. C. Ting, as presented in the 1998 proceedings of the American School of Gas Measurement Technology (1998, pp 85-88), Dr. Ting recognizes that standard orifice meter measurement of gas flow in a gas stream can be affected by a small amount of liquid entrained in the orifice meter, thereby calling into question the accuracy of said technique, which is widely utilized and relied upon in the industry. Conversely, the present invention provides a relatively cost effective and reliable system to provide detailed analysis as to the amount and content of entrained liquid within a gas flow, allowing the user to compensate for said entrained liquids in discerning gas flow measurements, as well as providing a detailed inventory of the composition of entrained liquid of the gas flow.
It is therefore an object of the present invention to provide a sample conditioning assembly that provides a gas sample at the prevailing process gas source pressure and temperature condition, thereby preventing gas phase composition changes
It is a further object of providing a method of sampling a gas from a gas stream having entrained liquid, wherein the gas sample is maintained at the prevailing process gas source pressure and temperature condition, thereby preventing gas phase composition changes.
It is another object of the present invention to provide a system for sampling gas from a gas stream which is more accurate, cost effective, and more easily implemented and maintained than prior art systems.
Lastly, it is an object of the present invention to provide a method for accurately sampling a gaseous hydrocarbon stream having entrained liquid therein, in order to accurately discern the properties thereof.